Mountain pine beetles are thinning lodgepole pine stands on the eastern side of Rocky Mountain National Park.

Bark beetles have been in the forests for about 34 million years.

The mountain pine beetle, Dendroctonus ponderosae, is a native of western conifer forests, where it breeds predominantly in ponderosa and lodgepole pines and less frequently in the other pines.

For as long as biologists have been paying attention, mountain pine beetles have cycled between tiny, innocuous populations and raging epidemics that kill a moderate to high proportion of the adult pines. The endemic phase lasts four to eight decades, while an epidemic lasts from several years to a decade.

So, on the one hand, the current epidemic is not unusual -- mountain pine beetles have erupted into epidemics for millions of years. But on the other hand, the current epidemic is an order of magnitude greater than the second largest epidemic ever recorded, reaching from New Mexico to the Yukon Territory and from the Front Range to the San Gabriel Mountains in California. The pressing question is "Why?"

In 1953, John Marr (the laboratory at CU's Mountain Research Station bears his name) established a series of long-term meteorological stations. The C1 station at 10,000 feet on Niwot Ridge provides clear evidence for what most of us can plainly see -- the world is getting warmer. More specifically, if the data are examined for temperatures allowing bark beetle development, the data show that spring temperatures permitting beetle development have increased 58 percent in the last four decades.

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As temperatures rose, mountain pine beetles extended their range. In the 1970s and 1980s, the Forest Service monitored beetle populations up to 9,000 feet, because the mountain pine beetle did not kill trees above 9,000 feet. Today, just 25 years later, the beetle is killing trees above 11,000 feet. At the same time, and in response to climate change, the beetle moved several hundred miles farther north in Canada.

In early June of 2008, my PhD. student Scott Ferrenberg and I saw beetles flying and attacking trees at 10,000 feet on Niwot Ridge. Historically, mountain pine beetles were first seen in late July and their peak flight was during the second and third week in August. Early June flight was such a departure from the historic norm that we decided to monitor bark beetle flight and development for the next two summers.

Historically, the bark beetle flight season lasted about 50 days. In 2009 and 2010, the flight seasons exceeded 120 and 130 days, respectively. The first beetles were seen or their pitch tubes were recorded in late May and early June, nearly two months earlier than expected.

Beetles flying in early June find a suitable tree and start excavating an egg gallery in about a day. A few days later, the first eggs are laid in the gallery. These early eggs hatch into larvae, which develop quickly in summer temperatures, emerging as adults in August. This summer generation has never been seen before in mountain pine beetles. Some beetle lineages are now able to fit two generations into a year, one from June to August and the next from August to June.

Historically, a female beetle laid about 60 eggs in August and one year later, under the best of conditions, 60 offspring emerged as adults. Today, a female starting in June can have 60 offspring by August, and each of those, under the best of conditions, can give her 60 grandchildren. So in one year a female can now produce 60 offspring and 3,600 grandchildren. While 60 offspring would rarely, if ever, be achieved due to the efforts of myriad predators, nevertheless two generations per year provides an exponential increase in the number of beetles flying and attacking trees.

The growing season in the Midwest is two to three weeks longer than it had been. Most of the glaciers in Glacier National Park have disappeared. Hundreds of species are extending their ranges, reaching higher in the mountains and colonizing new territory to the north. Like many other species, the mountain pine beetle is extending its range, is active earlier in summer and develops more quickly.

Although change in the mountain pine beetle is more apparent because they must kill a tree to reproduce, its response to our warming climate is not extraordinary.

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